Pulping of cellulose materials in the presence of free sulfur in a kraft pulping system and cyclic liquor recovery therefor



Oct. 5,

Filed April 8, 1963 23 22 F I g. l l SULFUR BURNER Wood M Sreum /2O REACTOR A l9 2 I k 7 h- FURNACE To DIGESTER s u k EVAPORATOR CARBONATOR STRIPPER Green -48 P Liquor 9 V w r V I0 DISSOLVER CLARIFIER WhiTe 28 Sludge Liquor SETTLER CAUSTICIZER Lime INVENTORS. LIME JAMES J. FERR|GAN,JR. KILN BY SYDNEY COPPlCK 1965 J. J. FERRIGAN, JR.. ETAL 235 PULPING' 0F CELLULOSE MATERIALS IN THE PRESENCE OF FREE SULFUR IN A KRAFT PULPING SYSTEM AND CYCLIC LIQUOR RECOVERY THEREFOR 2 Sheets-Sheet l AGENT Oct 1965 J. J. FERRIGAN, JR.. ETAL 3,210,235

PULPING OF CELLULOSE MATERIALS IN THE PRESENCE OF FREE SULFUR IN A KRAFT PULPING SYSTEM AND CYCLIC LIQUOR RECOVERY THEREFOR Filed April 8, 1963 2 Sheets-Sheet 2 SULFUR TOTAL SULFUR 6.5% T0 8.0% BASED ON DRY wooo P LP PULP F/g Z U BLACK LIQUOR H2O EVAPORATOR FLUE GAS HEATe- F R ACE U N PRIMARILY co SMELT oIssoLvER NA S NAZCO3 SLUDGEF CLARIFIER CARBONATOR TO FREE SULFUR CO I AS H18 cAUsTIcIzER CuCO N028 GASES NQOH H S 2. NAZCOR STRIPPER STEAM I 1 90C.

WHITE sETTLER LIQUOR CCLCO3 NE 8 z 1 NllH 3s ZHZO 350C; AL2O3 CAT LIME KILN LCCLO SULFUR so BURNER 1 %SULFUR INVENTORS.

JAMES J. FERRIGAN BY SYDNEY COPPICK EREUL ATTORNEY United States Patent G i PULPING F CELLULOSE MATERIALS IN THE PRESENCE OF FREE SULFUR IN A KRAFT PULPING SYSTEM AND CYCLIC LIQUOR RE- COVERY THEREFOR James J. Ferrigan, Jr., Chester, and Sydney Coppick, Ridley Park, Pa., assignors to Scott Paper Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Apr. 8, 1963, Ser. No. 271,357 4 Claims. (CL 16230) This invention relates to an improved pulping process dependent upon a particular pulping liquor recovery method. More particularly it relates to an improvement in the sulfate process for the preparation of wood pulp wherein wood is pulped in the presence of certain liquors and to the process of recovery of chemicals from these liquors to continue the cyclic process for the pulping of successive batches of wood.

It is well known in the art that alkaline liquors comprising a mixture of essentially sodium hydroxide and sodium sulfide solution are employed to dissolve the ligneous, resinous, or other encrusting components of wood chips or other fiber-bearing raw cellulosic material and to liberate the fibers as pulp suitable for papermaking or other purposes. After the cooking or digesting operation, which is carried out at elevated temperatures and pressures, the residual liquor is separated and washed from the remaining cooked wood pulp, which consists mainly of cellulosic fibers. The residual liquor contains most of the original inorganic constituents of the cooking liquor and the dissolved organic material from the raw chips or fiber-bearing material.

In order to recover the chemicals so that they may be further utilized for successive cooking of wood, they must first be separated from the solution mixture of chemicals and dissolved organic constituents of wood. This is commonly done by evaporating sufficient quantity of water from the residual black liquor so that it will support combustion. The evaporated liquor, which is usually 55-65% solids, is burned in a furnace where heat is recovered from the stack gases to produce process steam, and the hot smelt is allowed to pour from the furnace door into a dissolving tank. The furnace smelt contains the recovered sodium salts in the form of sodium sulfide and sodium carbonate.

As the dissolved sodium carbonate has little effectiveness in dissolving the ligneous and extraneous organic constituents of wood chips, it must be converted to sodium hydroxide which is an effective cooking chemical. In order to do this, the smelt solution which is called green liquor is causticized with lime. The sodium carbonate is converted as follows:

The precipitated limestone, CaCO is filtered and washed. The residual liquor or white liquor consisting of sodium sulfide and sodium hydroxide solution is suitable for the cooking of another batch of wood chips. The precipitated, drained limestone is heated in a lime kiln to remove carbon dioxide and produce quicklime which is employed in the causticizing step.

The outstanding feature of this alkaline pulping process, commonly called the kraft process, is the cyclic recovery of cooking chemicals. This recovery is so effective that between and of the chemicals in the residual spent cooking liquor is recovered for use in subsequent cooks. The tremendous economic advantage of chemical recovery has made the kraft process by far the most common of the pulping processes. In fact, any new pulping process must, in addition to possible yield and fibercharacteristic advantages over the kraft process, incorporate cooking chemical recovery if it .is to compete on an economic basis with the kraft process. In mill practice, sulfur is often added to the cooking liquor in quantities suflicient to make up the losses from the recovery cycle. In this invention, sulfur is added in quantities over and above the amount required for makeup in order to enjoy the yield advantage of the novel recovery process. This requires that sulfur be recovered in its elemental form under controllable conditions to maintain an eifective cyclic recovery process.

Even though the prior art kraft processes have tied in chemical recovery steps, these processes have not been able to achieve higher yields which are theoretically possible. Now methods of squeezing out increased pulp yields have not been successful since no effective and at the same time economical recovery system could be tied in with the attempts to increase yields.

The present process overcomes some of the prior art shortcomings in that higher fiber yields are possible in conjunction with a substantially complete chemical recovery cycle. Thus, the present method of pulping and recovery allows novel optimization of the yields by use of elemental sulfur being added to kraft cooking liquor to produce considerably higher pulp fiber yields than previously obtainable by the known kraft process.

The present invention is accomplished by improving the sulfate pulping process wherein the addition of excess elemental sulfur to the cooking liquor is used to produce a higher pulp yield while simultaneously recovering said sulfur in its elemental form to maintain a substantially complete recovery cycle.

The present invention is further accomplished whereby in a process of pulping cellulose-containing materials higher yields of pulp are obtained, the invention involving the continuous recovery of a liquor for a cyclic liquor recovery system comprising the steps of: introducing free sulfur into a digesting zone containing white liquor and cellulose-containing material, burning the spent liquor from the digesting zone to obtain green liquor separating the green liquor into two streams, introducing first separated green liquor stream into a carbonating zone into which zone carbon dioxide is present, stripping hydrogen sulfide containing gases from the carbonating .zone, combining the liquor from the stripping zone into the second separated green liquor stream, adding calcium oxide to the combined liquor stream from the hydrogen sulfide stripping zone and the green liquor stream, causticizing the calcium hydroxide containing stream, separating the sludge from the liquor of the causticizing step, Washing and burning the sludge from the causticizing step to recover calcium oxide for introducing it into the combined green liquor-hydrogen sulfide free stripping zone liquid, converting the hydrogen sulfide from the hydrogen sulfide stripping zone into free sulfur, combining the free sulfur with the sludge free liquor coming from the causticizing step and introducing the combined stream into the digesting zone.

In the more particular aspect, the invention is accomplished whereby in the improved pulping process, the following steps are carried out: introducing first separated green liquor stream into a carbonating zone, taking carbon dioxide in the form of a flue gas from the burning stage and introducing into the carbonating zone, converting the sodium sulfide into sodium carbonate and hydrogen sulfide, stripping the hydrogen sulfide from the liquid efiluents coming from the carbonating zone, recombining the liquid effluents from the carbonating zone with the second separated green liquor stream, taking the hydrogen sulfide and introducing it into a free sulfur reducing zone, recovering the liquid sulfur from the reducing zone and separating it into two streams, taking one liquid sulfur stream and burning it to produce sulfur dioxide, collecting and introducing the sulfur dioxide into the hydrogen sulfide reducing zone, taking the elemental sulfur, dissolving and combining it with the white liquor, and introducing the combined white liquor and elemental sulfur into the digesting zone.

With the foregoing in mind, the present invention can be comprehended more readily by reference to the attached drawing wherein the same reference characters as used herein indicate the process zones, steps and streams, and wherein:

FIGURE 1 represents a schematic drawing of the flow diagram illustrating the novel process, and;

FIGURE 2 represents a descriptive flow diagram illustrating the in and out streams and the interrelation of various liquor component recovery steps.

To more fully understand the invention, the following concepts should be kept in mind. In FIGURE 1, the green liquor 12 containing sodium carbonate and sodium sulfide has a larger ratio of sodium sulfide to sodium carbonate than the liquor from a normal kraft operation. This concept is a direct result of the fact that additional sulfur was initially employed in the cook.

In reference to the figure, the process is carried out as follows. After the digesting zone 1 has been filled with wood chips or other suitable cellulose-containing material and the cooking liquor added, elemental sulfur 2 in a suitable form is introduced into the digesting zone. As one aspect of the novel process, the process overcomes the inherent difiiculty in recovery of elemental sulfur by a novel method of introducing the sulfur into the white liquor. Normally, when liquid sulfur between 250 and 315 F. is poured into a cooler liquid, the rapid cooling transforms the sulfur into a pseudo-plastic, rubbery mass which is very difiicult to dissolve. This pseudoplastic form is eliminated by injecting the liquid sulfur into white liquor which is at the same temperature as the liquid sulfur. By heating the white liquor under pressure to the liquid sulfur temperature, and spraying the sulfur into the liquor, the sulfur dissolves readily. A smaller side stream of the white liquor may be separated and used for this purpose. The amount of sulfur used is from about 3.5 to about 5.0% based on dry wood. After the cook, the pulp 3 is separated from cooking liquor 4. The liquor is further treated in an evaporation zone 5 in order that it may be burned in a burning zone 6, to produce flue gas 7 which is needed in later stages of the process. The smelt 8 from the burning zone expressed as Na s and Na CO is sent to smelt dissolving zone 9 and from there to clarifying zone 10 from which sludge 11 is removed. The green liquor 12 is separated in two streams. Stream 13 is sent to further processing and it is from about 35 to about 45% of the original green liquor stream 12. The remainder makes up stream 14 which is introduced into the carbonating zone 15 together with the flue gas 7 consisting of CO H O, N and 0 from the burning zone 6 which flue gas effects the conversion of the bound sulfur in the green liquor to hydrogen sulfide. The carbonat-or operates at about 30 C. and 70 grams/liter Na O equivalent. The efiluent is at a pH of about 9. Hydrogen sulfide is stripped from the effluent stream 16 in hydrogen sulfide stripping zone 17 in presonce of steam at about C. Stream 18 contains Na CO and is recombined with the first green liquor stream 13. The components in the two combined streams amount to about 75% for Na CO and about 25% Na s. The hydrogen sulfide gas containing stream 19 is introduced into a hydrogen sulfide reducing zone 20. The reduction is commonly effected in a Claus reactor or some other type of reactor performing the function of converting the hydrogen sulfide in the presence of sulfur dioxide to liquid sulfur. The conversion is generally carried out in presence of suitable catalysts such as bauxite, hydrated alumina or Al O -2H O. The conversion is carried out at about 350 C. The liquid sulfur stream 21 is divided into two streams. The first stream 22 is sent directly to the pulping zone 1 or recombined with the pulping liquor before the pulping liquor is introduced into the pulping zone. The second stream 23 consisting of about 30% of stream 22 is sent to sulfur burning zone 24 where it is converted to sulfur dioxide; this sulfur dioxide stream 25 is the source for the reducing of hydrogen sulfide in zone 20. Returning. to streams 13 and 18; these streams making up the green liquor are combined in stream 26 which is treated with lime 27, in liming'zone 28 to produce CaCO Na S, and NaOH. Thereafter, the stream is introduced in settling zone 29 to produce the white liquor stream 30 which is the pulping liquor used in the novel process when combined with the liquid sulfur from stream 22.

As seen from above, the process depends on the chemical change effected on the various streams at various points in the process. For example, the fiue gas inter acts with a certain amount of green liquor. The green liquor is recombined with a stream that has been carbonated while the hydrogen sulfide interacts with sulfur dioxide which in turn yields sulfur. The interaction of sulfur with the white liquor and use of this liquor produces the improved yields. Further, the process allows easy modification of conditions as the process streams are easily monitored and modified to suit the process conditions. Thus, the amount of sulfur can easily be added to compensate for loss, while the green liquor can be made up along the process route directed to the white liquor.

In order to more fully understand this invention, the following discussion compares the known prior art with the present process and more particularly points out the novel process.

In the usual well-known cyclic sulfate pulping process, it is customary to replenish the losses of chemicals. These losses generally take place during the pulp Washing and further during the liquor recovery cycle.

The most common method of replenishing losses is via the addition of make-up chemicals prior to the introduction of black liquor into the reducing furnace. Usual-ly it is done 'by adding the chemicals in the form of salt cake or sodium sulfate; during the burning of the black liquor, the salt cake is reduced to sodium sulfide and carbonate, thus replenishing and maintaining the system in chemical balance, with regard to both sodium and sulfur requirements.

The sulfate addition is normally from about to about pounds of salt cake per ton of pulp. The amount depends, of course, upon the efiiciency of the recovery cycle. The added chemicals are equivalent to a make-up addition of sulfur amounting to from about 0.5 to about 0.75% sulfur on a dry wood basis.

In fact, in normal practice free sulfur is sometimes added directly in these quantities, together with either caustic or soda ash equivalents. This addition is in lieu of the salt cake make-up.

The present invention contemplates the use of makeup sulfur and sodium by means similar to the usual methods described above. However, over and above the normal additions to replace losses, the present invention contemplates the use of an extra 3.5 to 5% addition of sulfur (dry wood basis) to the digestion liquors.

To distinguish more .clearly between the usual kraft cycle and the present invention, the following sulfur material balances are set out.

In the usual procedure utilizing 20% chemicals on dry wood expressed as Na O, and 30% sulfidity, there will be present in the digester zone about 3% sulfur on dry wood, existing as NaHS and Na s. Thus we have: (basis dry wood) Percent Percent Percent Sulfur Used Sulfur Make-Up (In white Recovered Sulfur liquor) However, in the present invention, we have an entirely Thus it is apparent that the present invention contemplates sulfur additions far in excess of those normally added as replenishment to the system. Moreover, the additions are in fact larger than the total amounts of sulfur normally present in the digestion system.

These extra large amounts of sulfur exist, after the combustion stage, as sodium sulfide along with the sodium sulfide from the usual quantities of chemicals used. Thus a fraction of the green liquor, equivalent to 55 to 65% of the total, must be treated in a manner entirely different from the usual procedure to recover this additional sulfide in the form of elemental sulfur to perpetuate the cyclic process. The increase in yields, by employing dissolved sulfur, is dependent upon the amount of sulfur dissolved in the cooking liquor. In order to distinguish the present process from the normal kraft operation, this process is hereby defined as the sulfofate process.

A more detailed disclosure of this invention will be evident from the following illustrative examples.

EXAMPLE I A blend of 50% northeastern spruce and 50% northeastern balsam fir was prepared having an average chip length of A", and 1 part equivalent of dry wood was charged to the digester. Cooking liquor was added to the digester in the amount of 3.5 parts. The cooking liquor was a normal sulfate pulping liquor containing 20-2'l% chemical (as Na O) on basis of bone dry wood and having a sulfidity of 28-30%. The digester and its contents was heated via indirect steam and forced circulation so that the temperature was .raised to 172 C. in three hours and maintained at 172 C. for 1.5 hours, a maximum pressure .of 100 lbs/sq. in. was maintained. The temperature and pressure were reduced and the contents of the diagester dumped. The pulp was washed and screened and upon analysis gave the following results:

Yield of screen pulp percent 43.1

Yield of screenings do 2.7

Permanganate number 12.4

EXAMPLE II A blend of 50% northeastern spruce and 50% northeastern balsam fir digested as in Example I with the ex- 6 ception that 3.5% elemental sulfur was added (dry wood basis) to the starting liquor, and the digestion was carried out at 175 C. for 1.0 hour and at a maximum pressure of lbs/sq. in. The washed and screened pulp gave the following results:

Yield of screened pulp percent 47.4

Yield of screenings do 0.6

Permanganate number 12.8

EXAMPLE III Balsam fir was digested as in Example I with the exception that the digestion was carried out at C. for 1.0 hour and the maximum pressure was maintained at 110 lbs/sq. in. The washed and screened pulp gave the following results:

Yield of screened pulp percent 42.2

Yield of screenings do 1.1

Permanganate number 12.3 EXAMPLE IV Balsam fir was digested as in Example III with the exlception that 3.5% elemental sulfur (dry wood basis) was added to the starting liquor. The washed and screened pulp gave the following results:

Yield of screened pulp percent 45.6

Yield of screenings do 0.2

Permanganate number 13.9

EXAMPLE V Pond cypress (Taxodium ascendens B.) was digested as in Example I with the exception that the digestion was carried out at 175 C. for 1.0 hour at a maximum pressure of 110 lbs/sq. in. The washed and screened pulp gave the following results:

Yield of screened pulp percent 43.8

Yield of screenings do 0.15

Permanganate number 15.2

EXAMPLE VI Pond cypress was digested as in Example V with the .exception that 3.5% elemental sulfur (dry wood basis) was added to the starting cooking liquor. The washed and screened pulp gave the following results:

Yield of screened pulp percent 45.9

Yield of screenings do 0.4

Permanganate number 14.0

EXAMPLE VII A chip blend comprised of 80% northeastern softwood (i.e., spruce, balsam fir) and 20% northeastern hardwood (i.e., beech, birch, maple) and digested as in Example I, with the exception that the digestion was carried out for 1.0 hour. The washed and screened pulp gave the following results:

Yield of screened pulp percent 45.1 Yield of screenings do 0.1 Permanganate number 12.6

EXAMPLE VIII A chip blend similar to that employed in Example VII was digested as in Example VII with the exception that 3.5% elemental sulfur (dry wood basis) was added to the starting liquor. The washed and screened pulp gave the following results:

Yield of screened pulp percent 48.0

Yield of screenings do 0.3

Permanganate number 12.5

EXAMPLE IX A similar chip blend was digested as in Example VII with the exception that 5.0% elemental sulfur (dry Wood basis) was added to the starting liquor. The washed and screened pulp gave the following results:

3' Yield of screened pulp percent 50.5 Yield of screenings do 0.5 Permanganate number 14.2

It is obvious from the above that there is a 4.3% increase in yield by cooking the spruce-balsam mixture with 3.5% dissolved sulfur on wood. When 3.5% dissolved sulfur on wood is employed in Example VII which is an 80% softwood-20% hardwood mixture, the yield increase is 2.9%. These yield increases are maintained with approximately the same permanganate numbers, which are a measure of bleachability. By adding 5% dissolved sulfur on Wood when pulping as in Example IX, the yield is increased 5.4% with only a 2 point rise in permanganate number. A significant point is that these yield increases take place with no important changes in the plup properties.

These measurements were carried out according to the TAPPI standard tests. In a similar manner the yields of screened pulp, yield of screenings and permanganate number were carried out according to TAPPI standard tests.

What we claim is:

1. A process for pulping cellulose-containing materials in a kraft pulping system having a cyclic liquor recovery comprising the steps of: introducing free sulfur into a digesting zone together with white liquor and cellulosic materials, said sulfur being in excess of from about 28% to about 30% sulfidity of the white liquor by about 3.5 to about 5.0% based on the dry cellulosic material; pulping said cellulose materials; withdrawing from the digesting zone a black liquor after pulping the cellulosic mateerial and burning it to obtain green liquor and flue gas; carbonating by means of the flue gas of from about 55% to about 65% of the green liquor to obtain hydrogen sulfide and sodium carbonate; converting hydrogen sulfide to free sulfur combining the uncarbonated green liquor with the sodium carbonate from the carbonating step; caustioizing the combined stream of green liquor and sodium carbonate to obtain white liquor and; reintroducing the white liquor and the free sulfur derived from the obtained hydrogen sulfide into the digesting zone.

2. A process for pulping cellulosic materials in a kraft pulping system having a cyclic recovery system and 3.5% to 5% free sulfur based on dry wood, the process comprising the steps of: introducing into a digesting zone cellulosic materials, free sulfur and an aqueous liquor of sodium sulfide and sodium hydroxide, the weight of the free sulfur in the combined stream being of from about 3.5% to about 5% based on dry wood in excess of from about 28% to about 30% sulfidity of the liquor; digesting said cellulosic materials with the liquor in the digesting zone to obtain pulp and a black liquor; obtaining green liquor and flue gases by burning the black liquor separated from the pulp; separating the green liquor into a first stream .and a second stream; introducing the first separated stream which is of from about 55% to about 65% of the total green liquor stream into a carbonating zone and introducing flue gases from the burning step into the carbonating zone to elfect carbonation of the first green liquor stream to obtain hydrogen sulfide and sodium carbonate; separating by steam stripping the efiiuent from the carbonating zone into a hydrogen sulfide-containing stream and a sodium carbonate stream free from hydrogen sulfide; introducing the sodium carbonate stream and the second green liquor stream into a causticizing zone; causticizing the combined green liquor and sodium carbonate by introducing into it calcium and magnesium oxide and settling the sludges therein to obtain white liquor; converting the separated hydrogen sulfide in a reducing zone to obtain free sulfur; recovering free sulfur; and reintroducing as the aqueous liquor the White liquor and the free sulfur into the digesting zone.

3. A process for pulping cellulosic materials in presence of free sulfur in a kraft pulping system having cyclic pulping liquor recovery, the process comprising the steps of: introducing into .a digesting zone free sulfur from 3.5% to about 5% based on bone-dry cellulosic material, said zone containing white liquor and cellulosic material; pulping said cellulosic material; separating spent liquor from the pulp and burning the liquor to obtain green liquor and carbon dioxide; separating the green liquor into two streams, the first of which is from about 55 to about of the total amount of green liquor; introducing the first separated green liquor stream into a carbonating zone together with the carbon dioxide obtained from burning the spent liquor to effect carbonation; stripping in a stripping zone, by means of steam, hydrogen sulfite gas from the carbonated stream to obtain a stripped liquor; combining the liquor from the stripping zone with the second separated green liquor stream; causti cizing with calcium oxide the combined liquor streams from the hydrogen sulfide stripping zone and the second green liquor stream, and settling CaCO as the sludge; reducing in presence of sulfur dioxide the hydrogen sulfide to free sulfur, said hydrogen sulfide being obtained from the stripping zone; reintroducing the causticized and settled liquor stream which is at about 28% to about 30% sulfidity and the free sulfur into the digesting zone, said free sulfur being present in an amount of from about 3.5% to about 5% based on the weight of dry cellulosic material, and which makes up with sulfur in the white liquor of from about 5% to about 8% total sulfur based on the weight of dry cellulosic material.

4. A process for pulping cellulosic materials according to claim 3 comprising the steps of: reducing the hydrogen sulfide to free sulfur in presence of sulfur dioxide and alumina at about 350 C. in a hydrogen sulfide reducing zone; recovering liquid sulfur and separating said sulfur into a first stream and a second stream; burning the first liquid sulfur stream to produce sulfur dioxide; introducing the sulfur dioxide into the hydrogen sulfide reducing zone; dissolving and combining the second sulfur stream in the amount of from about 3.5 to about 5% based on the dry weight of the cellulosic material to be pulped with white liquor and introducing the combined white liquor and elemental sulfur into the digesting zone.

References Cited by the Examiner UNITED STATES PATENTS 1,898,613 2/33' Bradley et al. 162-30 2,099,400 11/37 Jones 16282 2,490,078 12/49 Meiler 16282 2,705,187 3 /55 Terziev 23-13 8 2,944,928 7/60 Kibrick et a1. 16282 OTHER REFERENCES Chemical Abstracts, Cooking of Sulfate Pulp with the Addition of Sulfur, vol. 40, No. 21, Nov. 1946 (68127).

Kress: Development of High Sulphidity in Kraft Cooking Liquors, Paper Trade Journal, vol. 119, No. 17, Oct. 26, 1944, pages 4549.

Sulphur in Soda Pulp Cooking, Special Inquiry No. 14, TAPPI, Technical Association Special Reports, December 1926, pages 61-67.

Use of Sulphur in Preparation of Caustic Cooking Liquors (Abstracts of Patents), Paper Trade Journal, Aug. 4, 1927, page 62.

DONALL H. SYLVESTER, Primary Examiner.

MORRIS O. WOLK, Examiner. 

1. A PROCESS FOR PULPING CELLULOSE-CONTAINING MATERIALS IN A KRAFT PULPING SYSTEM HAVING A CYCLIC LIQUOR RECOVERY COMPRISING THE STEPS OF: INTRODUCING FREE SULFUR INTO A DIGESTING ZONE TOGETHER WITH WHITE LIQUOD AND CELLULOSIC MATERIALS, SAID SULFUR BEING IN EXCESS OF FROM ABOUT 28% TO ABOUT 30% SULFIDITY OF THE WHITE LIQUOR BY ABOUT 3.5% TO ABOUT 5.0% BASED ON THE DRY CELLULOSIC MATERIAL; PULPING SAID CELLULOSE MATERIALS; WITHDRAWING FROM THE DIGESTING ZONE A BLACK LIQUOR AFTER PULPING THE CELLOLOSIC MATEERIAL AND BURNING IT TO OBTAIN GREEN LIQUOR AND FLUE GAS; CARBONATING BY MEANS OF THE FLUE GAS OF FROM ABOUT 55% TO ABOUT 65% OF THE GREEN LIQUOR TO OBTAIN HYDROGEN SULFIDE AND SODIUM CARBONATE; CONVERTING HYDROGEN SULFIDE TO FREE SULFUR COMBINING THE UNCARBONATED GREEN LIQUOR WITH THE SODIUM CARBONATE FROM THE CARBONATING STEP; CAUSTICIZING THE COMBINED STREAM OF GREEN LIQUOR AND SODIUM CARBONATE TO OBTAIN WHITE LIQUOR AND; REINTRODUCING THE WHITE LIQUOR AND THE FREE SULFUR DERIVED FROM THE OBTAINED HYDROGEN SULFIDE INTO THE DIGESTING ZONE. 